Refactor TimeSeries artifacts (#651)

* Reindent TimeSeries classes

* Fix spacing in time series tests

* Remove trend tests where current is nil

I think if we've gotten this far with a nil value for current, there's a data integrity problem.

If we allow this, we'll have to be very defensive in our code. Best to raise and fix early.

* Reindent Money class

* Refactor TimeSeries artifacts

* Use as_json in TimeSeries

* Bring back tests for trends where current is nil

* Bring back trend test

* Correctly enumerate trend test

* Use favorable_direction for trend_styles helper

* Make trend public in TimeSeries::Value

* Allow nil current values in trends

I think I might've gotten it wrong before, nils might appear in trends if values are unavailable for snapshots

* Clean up TimeSeries::Trend

* Skip trend values same class validations if any values are nil

* Refactor Money

* Remove object parsing in TimeSeries::Value

We're only every passing hashes

app/views/shared/_value_heading.html.erb

@@ -16,7 +16,7 @@
   </p>
   <% if trend.nil? %>
     <p class="text-sm text-gray-500">Data not available for the selected period</p>
-  <% elsif trend.direction == "flat" %>
+  <% elsif trend.direction.flat? %>
     <p class="text-sm text-gray-500">No change vs. prior period</p>
   <% else %>
     <div class="flex items-center gap-2">